Background: In the “island of inversion,” ground states of neutron-rich $sd$-shell nuclei exhibit strong admixtures of intruder configurations from the $fp$ shell. The nucleus ${}^{30}\mathrm{Mg}$, located at the boundary of the island of inversion, serves as a cornerstone to track the structural evolution as one approaches this region.

Purpose: Spin-parity assignments for excited states in ${}^{30}\mathrm{Mg}$, especially negative-parity levels, have yet to be established. In the present work, the nuclear structure of ${}^{30}\mathrm{Mg}$ was investigated by in-beam $\gamma$-ray spectroscopy mainly focusing on firm spin-parity determinations.

Method: High-intensity rare-isotope beams of ${}^{31}\mathrm{Mg}$, ${}^{32}\mathrm{Mg}$, ${}^{34}\mathrm{Si}$, and ${}^{35}\mathrm{P}$ bombarded a Be target to induce nucleon removal reactions populating states in ${}^{30}\mathrm{Mg}$. $\gamma$ rays were detected by the state-of-the-art $\gamma$-ray tracking array GRETINA. For the direct one-neutron removal reaction, final-state exclusive cross sections and parallel momentum distributions were deduced. Multinucleon removal reactions from different projectiles were exploited to gain complementary information.

Results: With the aid of the parallel momentum distributions, an updated level scheme with revised spin-parity assignments was constructed. Spectroscopic factors associated with each state were also deduced.

Conclusions: Results were confronted with large-scale shell-model calculations using two different effective interactions, showing excellent agreement with the present level scheme. However, a marked difference in the spectroscopic factors indicates that the full delineation of the transition into the island of inversion remains a challenge for theoretical models.

• Received 25 June 2020
• Accepted 9 October 2020

DOI:https://doi.org/10.1103/PhysRevC.102.054318